Field of the Invention
The present invention relates to SiC sublimation crystal growth.
Description of Related Art
Wafers of silicon carbide of the 4H and 6H polytype serve as lattice-matched substrates to grow epitaxial layers of SiC and GaN, which are used for fabrication of SiC- and GaN-based semiconductor devices for power and RF applications.
With reference to FIG. 1, large SiC single crystals are commonly grown by the technique of Physical Vapor Transport (PVT). FIG. 1 shows a schematic view of a typical PVT growth cell, wherein PVT growth of a SiC single crystal 15 is carried out in a graphite crucible 11 sealed with a graphite lid 12 and loaded with a sublimation source 13 disposed at a bottom of crucible 11 and a single crystal SiC seed 14 disposed at the crucible top. Sublimation source 13 is desirably polycrystalline SiC grain synthesized in a separate process. Loaded crucible 11 is placed inside of a growth chamber 17 where it is surrounded by thermal insulation 18. Inductive or resistive heating is used to bring crucible 11 to a suitable temperature, generally, between 2000° C. and 2400° C., for the PVT growth of a SiC single crystal 15 on SiC single crystal seed 14.
FIG. 1 shows a typical inductive heating arrangement with a RF coil 19 placed outside growth chamber 17, which is desirably made of fused silica. RF coil 19 is positioned with respect to crucible 11 such that during growth of single crystal 15, a temperature of sublimation source 13 is maintained higher than a temperature of the seed crystal 14, typically, by 10° C. to 200° C.
Upon reaching suitable high temperatures, sublimation source 13 vaporizes and fills crucible 11 with vapor 16 of Si, Si2C and SiC2 molecules. The temperature difference between sublimation source 13 and seed crystal 14 forces vapor 16 to migrate and condense on seed crystal 14 thereby forming single crystal 15. In order to control the growth rate, PVT growth is carried out in the presence of a small pressure of inert gas, typically, between several and 100 Torr.
Generally, SiC crystals grown using this basic PVT arrangement suffer from numerous defects, stress, and cracking. To this end, it is difficult to grow long boules of SiC single crystal 15 using conventional PVT due to carbonization of sublimation source 13 and subsequent massive incorporation of carbon inclusions in single crystal 15. Cracking becomes a major yield loss when the conventional PVT technique is utilized to grow large-diameter SiC single crystals.
Inclusions in PVT-grown crystals, e.g., single crystal 15, include carbon inclusions (particles), silicon droplets, and foreign polytypes. Carbon particles in single crystal 15 can be traced to SiC sublimation source 13 and the graphite forming crucible 11. Specifically, silicon carbide sublimes incongruently producing a silicon-rich vapor and carbon residue in the form of very fine carbon particles. During growth of single crystal 15, these fine particles become airborne and, transferred by the flow of vapor 16, incorporate into growing single crystal 15. Massive carbon incorporation into single crystal 15 happens at the end of the growth of single crystal 15 when a large amount of carbon residue is present in crucible 11.
Vapor erosion of the graphite forming crucible 11 can also produce carbon inclusions. During growth, the inner walls of crucible 11 are in contact with Si-rich vapor 16 which attack the graphite forming crucible 11 and erode it. Structurally, the graphite forming crucible 11 includes graphitic grains embedded into the matrix of graphitized pitch. The graphitized pitch is attacked by vapor 16 first. This leads to liberation of graphite grains which are transferred to the growth interface of single crystal 15.
Silicon inclusions (droplets) usually form at the beginning of the growth of single crystal 15, when the SiC sublimation source 13 source is fresh. Vapor 16 over SiC sublimation source 13 can contain a too high fraction of silicon, which can cause the formation of Si liquid on the growth interface of single crystal 15 and incorporation of Si droplets into single crystal 15.
A large number of polytypic modifications of silicon carbide exist, and inclusion of foreign polytypes in sublimation-grown 4H and 6H single crystal 15 is common (15R inclusions are most frequent). The origin of polytypic inclusions is often tied to the appearance of macrosteps on the growth interface of single crystal 15. The facets formed on the macrosteps are not stable against stacking faults. These stacking faults latter evolve during growth of single crystal 15 into foreign polytypes in single crystal 15.
Two technological factors affect the stability of the 6H and 4H polytypes during growth of single crystal 15. One is the curvature of the growth interface of single crystal 15. A flat or slightly convex growth interface of single crystal 15 is believed to be more stable against polytypic perturbations than a more curved interface, convex or concave. Another factor is the stoichiometry of vapor 16. It is believed that stable growth of the SiC crystals 15 of hexagonal 4H and 6H polytypes requires a vapor phase enriched with carbon, while a too high atomic fraction of Si in the vapor can lead to the appearance of foreign polytypes.
Three types of dislocations can generally exist in SiC single crystal 15 grown by PVT: threading screw dislocations, threading edge dislocations, and basal plane dislocations. The lines of the threading dislocations tend to position along the crystallographic c-direction, which is often used as a growth direction of SiC single crystals 15. Basal plane dislocations are dislocations with their lines parallel to the basal c-plane.
A micropipe is a threading screw dislocation with a large Burgers vector. When the Burgers vector exceeds (2-3)·c, the crystal relieves the stress caused by the dislocation by forming a hollow core, from a fraction of a micron to 100 microns in diameter.
Upon nucleation, growing SiC single crystal 15 inherits some of the dislocations from seed crystal 14. During growth of SiC single crystal 15, micropipes and dislocations participate in reactions with other micropipes and dislocations. This leads to a progressive reduction in the micropipe/dislocation densities during growth. In the case of growth disturbance, such as incorporation of a carbon particle or foreign polytype, new micropipes and dislocations are generated.
It has been observed that the magnitude of growth-related stress increases with the increase in the length and diameter of a SiC single crystal boule formed by the growth of SiC single crystal 15. More specifically, SiC single crystal 15 grown by conventional PVT exhibits nonuniform thermo-elastic stress and its shear component often exceeds the critical value of 1.0 MPa leading to plastic deformation. Plastic deformation occurs via generation, multiplication and movement of dislocations. Unresolved stress accumulated during growth of a boule of SiC single crystal 15 can lead to cracking of the boule formed by the growth of SiC single crystal 15 during cooling of said boule to room temperature or during subsequent wafer fabrication.
With reference to FIG. 2, since the inception of the PVT growth technique, a number of process modifications have been developed. In one such modification, a cylindrical, gas-permeable divider 25, made of either thin-walled dense graphite or porous graphite, is utilized to divide a crucible 20 into two concentric compartments: a source storage compartment 24 containing a solid SiC sublimation source material 21 and a crystal growth compartment 26 with a SiC single crystal seed 22 at the bottom. For the purpose of simplicity, an RF coil and a growth chamber have been omitted from FIG. 2.
At high temperatures, SiC sublimation source 21 vaporizes and vapor 27 fills compartment 24. The volatile Si- and C-bearing molecules in vapor 27 diffuse across divider 25 and enter crystal growth compartment 26, as shown by the arrows in FIG. 2. Then, driven by the axial temperature gradient, vapor 27 migrate downward to SiC single crystal seed 22 and condense on it causing growth of a SiC single crystal 23.
The PVT process shown and described in connection with FIG. 2 has drawbacks, including, without limitation, the nucleation of polycrystalline SiC on the graphite walls of crucible 20 and/or divider 25, the nucleation of polycrystalline SiC on the edges of SiC single crystal seed 22, and a high degree of stress in the grown SiC single crystal 23. This PVT modification is considered inapplicable to the growth of industrial size SiC boules.
With reference to FIG. 3, in another modification of the basic PVT growth technique, PVT is used in combination with High Temperature Chemical Vapor Deposition (HTCVD) to achieve continuous growth of SiC single crystals of unlimited thickness. In the schematic diagram of a Continuous Feed PVT process (CF-PVT) shown in FIG. 3, a crystal growth crucible 30 is divided into two chambers: a lower chamber 33 for the HTCVD process, and an upper chamber 34, which includes a SiC single crystal seed 36, for PVT. Chambers 33 and 34 were separated by one or more members 35 made of gas-permeable graphite foam. Solid SiC source material 39 is placed atop the upper surface of foam member 35 that faces SiC single crystal seed 36. Heating of SiC source material 39 is provided by an RF coil 31 coupled to a graphite susceptor 32 in a manner known in the art.
Gaseous trimethylsilane (TMS) 37 is supplied to lower chamber 33 assisted by a peripheral flow of argon 38. At high temperatures, the TMS molecules undergo various chemical transformations. The gaseous products of these transformations diffuse through foam member 35 and form solid SiC, either in the bulk of foam member 35 or on the upper surface of foam member 35. In upper chamber 34, a conventional PVT growth process takes place. Namely, solid SiC source material 39 sublimates, its vapor migrates to SiC single crystal seed 36 and condenses thereon causing growth of SiC single crystal 36′.
It was believed that gas-feeding through foam member 35 would prolong the life of the SiC source material 39 and prevent its carbonization. However, thick and/or long boules of SiC single crystal 36′ where unable to be grown due to the erosion of foam member 35, source carbonization, formation of graphite inclusions and other defects in the growing SiC single crystal 36′. For the purpose of simplicity, the growth chamber has been omitted from FIG. 3.
With reference to FIG. 4, another modification of the basic PVT growth technique includes a susceptor 46, a crucible 43 containing semiconductor purity silicon 42, a SiC seed 40 attached to a seed-holder 41, and a high-purity, gas-permeable membrane 47 disposed between seed 40 and silicon 42. Membrane 47 can be in the form of porous graphite disc or in the form of dense graphite disc with multiple holes.
Upon heating, silicon 42 melts and vaporizes. The Si vapor emanating from the molten silicon 42 diffuses through porous membrane 47, where it reacts with carbon of membrane 42 producing volatile Si2C and SiC2 molecular associates. Vapor 44 including the volatile Si2C and SiC2 molecular associates escape from membrane 47, migrate to seed 40, and condense on it causing growth of single crystal 45. Thus, membrane 47 serves as a source of carbon. For the purpose of simplicity, an RF coil and a growth chamber have been omitted from FIG. 4.
One of the shortcomings of prior art SiC sublimation growth techniques is the phenomenon of vapor erosion of graphite. With reference to FIG. 5, in conventional PVT growth a crystal growth crucible 50 includes solid a SiC source 51 at the bottom, a SiC seed 52 attached to the crucible top, and a SiC single crystal 54 growing on seed 52. Usually, the edge of the boule of SiC single crystal 54 is in close proximity to (sometimes touching) a graphite sleeve 55 disposed in the vicinity of the growing SiC single crystal 54. This sleeve 55 can be a heat shield, growth guide, or the crucible wall, all generally made of graphite. The distance between the SiC single crystal 54 and SiC source 51 is usually much more significant.
During growth of SiC single crystal 54, SiC source 51 sublimes and generates Si-rich vapor 53, with an Si:C atomic ratio generally between 1.1 and 1.6, and carbon residue 51a. Vapor 53 in the space 57 adjacent to the SiC source 51 is in equilibrium with the SiC+C mixture. Driven by the temperature gradient, vapor 53 moves axially toward SiC seed 52. This movement of vapor 53 is in the form of Stefan gas flow with the linear rate of about 1-10 cm/s.
Upon reaching the growth interface, vapor 53 condenses causing growth of the SiC single crystal 54. Precipitation of stoichiometric SiC from the Si-rich vapor 53 makes the vapor even more Si-rich in the space 58 adjacent SiC crystal 54. Therefore, the vapor phase composition in this space does not correspond anymore to the SiC+C equilibrium. Instead, vapor 53 is now in equilibrium with either SiC of a certain stoichiometry or, in the extreme case, with the two-phase SiC+Si mixture. A too high content of Si in vapor 53 can lead to the formation of the liquid Si phase on the growth interface and incorporation of Si droplets into the growing crystal.
The atomic fraction of Si in vapor 53 in space 58 is the highest inside crucible 50, and this forces excessive Si to diffuse out of space 58. Due to the significant distance between SiC single crystal 54 and SiC source 51 and the presence of the axial Stefan flow in crucible 50, the excessive Si does not reach SiC source 51. Rather, it diffuses from SiC single crystal 54 toward and reaches the nearest graphite part—sleeve 55. This diffusion is shown by arrows 56. This Si-rich vapor (which is not in equilibrium with carbon) attacks graphite sleeve 55 and erodes it producing SiC2 and Si2C gaseous molecules.
In a typical PVT geometry, the temperature of sleeve 55 is higher than that of the SiC single crystal 54. Driven by this radial temperature gradient, the gaseous products of graphite erosion (SiC2 and Si2C) diffuse back toward SiC single crystal 54, as shown by arrows 56a, and enrich space 58a in the peripheral area 54b of SiC single crystal 54 in front of the growth interface with carbon. In other words, a zone of vapor circulation emerges at the edges of SiC single crystal 54 with silicon acting as a transport agent and transporting carbon from sleeve 55 to the lateral regions of growing SiC single crystal 54. In SiC single crystals 54 grown by the PVT technique, carbon from sleeve 55 can comprise up to 20% of the total carbon content of the crystal.
The net result of this vapor circulation is the formation of two distinct regions in the vapor in the vicinity of the growing crystal. The vapor in central region 58 has a higher atomic fraction of silicon than the vapor in the lateral region 58a. Accordingly, central area 54a of SiC single crystal 54 grows from Si-rich vapor, while the peripheral area 54b of SiC single crystal 54 grows from the vapor containing a higher fraction of carbon.
Such compositional nonuniformity of the vapor phase has negative consequences for the crystal quality, including:                Spatial nonuniformity of the crystal composition (stoichiometry) resulting in a high degree of crystal stress, cracking and spatially nonuniform incorporation of impurities and dopants;        Formation of foreign polytypes and related defects;        Inclusion of carbon particles transported from the source;        Inclusion of carbon particles transported from the eroded sleeve; and        Inclusion of Si droplets in central areas of the crystal.        
For the purpose of simplicity, an RF coil and a growth chamber have been omitted from FIG. 5.